Hybrid vehicle system having engine with variable valve operation

ABSTRACT

A system for a vehicle, comprising of an internal combustion engine coupled in the vehicle, the engine having at least one cylinder with an intake and exhaust valve, where the opening and closing timing of the intake valve is adjustably retardable and the opening and closing timing of the exhaust valve is adjustably retardable, during engine operation, and an energy conversion device coupled in the vehicle capable of selectively supplying and absorbing torque during vehicle operation.

BACKGROUND AND SUMMARY

Engines have used various forms of variable cam timing to improve engineoperation over a variety of speed/load conditions. Further, hybridvehicle systems may use variable cam timing to improve overall vehicleoperation.

One approach to such a system is described in Japanese SAE paper9739552. This system described a hybrid electric vehicle (HEV) using anAtkinson engine with intake variable cam timing enabling late intakevalve closing during shutdown and cranking (120 deg after BDC). The lateintake valve closing may be used to reduce engine vibration duringengine restarts.

However, the inventors herein have recognized a problem with such anapproach. Specifically, Atkinson-cycle engines typically suffer afundamental disadvantage of poor torque at low to medium engine speeds.The reduced peak torque levels may then lead to secondary problems withnoise, vibration, and harshness (NVH) and fuel efficiency because higherengine speeds are required to produce sufficient power in real customerdriving.

Thus, in one approach, the above issues may be addressed by a system fora vehicle, comprising: an internal combustion engine coupled in thevehicle, the engine having at least one cylinder with an intake andexhaust valve, where the opening and closing timing of the intake valveis adjustably retardable and the opening and closing timing of theexhaust valve is adjustably retardable, during engine operation; and anenergy conversion device coupled in the vehicle capable of selectivelysupplying and absorbing torque during vehicle operation.

In this way, it is possible to obtain improved starting operation, forexample by utilizing both intake and exhaust retard. Further, it is alsopossible to obtain improved torque output during low to mid enginespeeds. In other words, late intake/exhaust valve timings may be used toreduce fresh air pumped through the engine during engine shutdown andcranking, thereby reducing oxygen flow to the catalysts in the exhaust.Further, such operation also may reduce NVH during engine starting(cranking) and/or shutdown operation. However, by having variable intakeand exhaust valve timing retard, wide-open throttle torque penaltiessuch as in the Atkinson cycle are reduced, and it is actually possibleto obtain some torque and power improvement.

Additionally, using both intake and exhaust valve timing retard reducesissues with late intake valve opening in a non-Atkinson cycle enginewith variable intake valve timing retard. For example, in such a case,the late intake valve opening may increase noise and vibration, andagain the valve timing adjustments may not assist in improving wide-openthrottle torque output. Thus, more advanced timings may be used at leastduring some higher torque output conditions to better take advantage ofthe hybrid propulsion system and obtain better overall vehicleperformance during real world driving conditions.

Furthermore, by using both intake and exhaust valve timing retard it ispossible to obtain improved fuel efficiency and feedgas emissions atpart throttle operating conditions, for example.

DETAILED DESCRIPTION OF THE FIGURES

FIG. 1 shows an example hybrid drive system;

FIGS. 2-3 show example engine systems with various variable valveoperation;

FIG. 3A shows example intake and exhaust valve timing with dual retardoperation;

FIGS. 4-7 are high level flow charts of example operation;

FIG. 8 is an example plot of signals that may be of interest during asimulated engine stop and start sequence;

FIG. 9 is an example plot of signals that may be of interest during analternate simulated engine stop and start sequence;

FIG. 10 is a flow chart of an example stopping sequence for a variableevent valvetrain engine; and

FIG. 11 is a flow chart of an example starting sequence for a variableevent valvetrain engine.

DETAILED DESCRIPTION

The present application relates to operation of an engine havingadjustable intake and exhaust valve operation in a hybrid propulsionsystem. While adjustable engine valve timing operation may be used toobtain various advantages in a hybrid propulsion system, there arenumerous constraints that have limited different performance aspects ofhybrid propulsion systems.

As one example, an Atkinson cycle may be used to provide improved fueleconomy and performance when matched with a hybrid propulsion system,however, the Atkinson cycle may result in degraded peak torque output atlow to mid speeds, thus potentially reducing vehicle performance undersome conditions, such as low battery state of charge. Further, the poortorque response may lead to secondary problems with NVH and efficiencybecause a higher RPM is required to produce sufficient power in realcustomer driving. One example of an Atkinson engine cycle is wherefairly late intake valve closing (IVC) timing is present (e.g., 92 degafter BDC). In another example, using Atkinson combined with Intake VCT,even later IVC (120 deg after BDC) may be used.

Thus, variable cam timing may be used to enable operation other than anAtkinson cycle, depending on the range of valve timing utilized. Typicalhydraulic variable cam timing actuators utilize a default position wheninsufficient hydraulic pressure is present to enable control, such asduring engine starting, which places still further limits on the advanceand retard of valve timing. Another possible approach may use intake VCTwithout an Atkinson cycle. Such a system could achieve late IVC, butthis would also entail very late IVO (Intake Valve Opening time). LateIVO means that both valves would be closed for the first part of theintake stroke, so the gases would be expanded to a vacuum until theintake valve opens. When the intake valve finally opened, air would rushinto the vacuum, potentially increasing induction noise. Also, the powerrequired to expand gases during the intake stroke and correspondingtorque pulsations on the crankshaft may decrease part or all of the NVHbenefit of late IVC. Furthermore, an intake VCT mechanism set up forthis purpose may not improve wide-open throttle operation thatconventional intake VCT is used for (with IVC of 120 deg after BDC, 60degrees of motion would not enable an early enough IVC for improved lowRPM torque).

As another example, in some hybrid systems, the engine is started andstopped at an increased frequency. However, the torque pulses generatedby the compression stroke during start-up and/or showdown may result inincreased noise and vibration, thus reducing drive feel. Furthermore,the repeated operation may result in increased fresh air, or oxygen,pumped through the exhaust system, potentially saturating emissioncontrol devices with oxygen and thus reducing performance during lateroperation. In other words, during shutdown and during cranking, fuel maybe off while the engine spins, so the engine pumps air into catalysts.After the engine starts, it may be run rich temporarily to purge oxygenfrom the catalyst and enable NOx reduction, however, some NOx emissionmay still occur, along with potentially increased hydrocarbons andreduced fuel economy.

Still further examples are present, as will be described in more detailherein. However, one approach that may be used to address at least someof these issues is to utilize a dual retard strategy, in which variationof valve timing is used to provide substantial retard of both intake andexhaust events. As will be described in more detail below, use of a dualretard strategy in a hybrid propulsion system also includesconsideration of default timing, such as caused by a default hydraulicactuator location, and appropriate selection of the default timing.

With such a system, it is possible to utilize dual retard operation toobtain significant fuel economy and emissions benefits at part load, andsome benefits at wide-open throttle, by utilizing varying amounts ofretard depending on operating conditions. Further, such operation may beused to improve engine starting, cranking, and shutdown.

For example, the table below summarizes various conditions and indicateshow such a system may be used to advantage in which the default timingis selected to be at a retarded timing, such as fully retarded. Forexample, if the valve timing may be varied in a range between, thedefault position may be selected to be in a later half of the range.

In one example, substantially late IVC (Intake Valve Closing time)during engine shutdown and during cranking is utilized to reduce theflow of oxygen to the exhaust system, and reduce compression torquepulses. Specifically, the substantially late IVC attained with dualretard may reduce the volumetric efficiency of the engine, which reducesairflow to the exhaust catalyst during engine shutdown and re-start.Further, a lower air charge trapped in the cylinders and reducedeffective compression ratio also reduces torque pulsations on thecrankshaft during cranking due to compression/expansion work. This mayprovide reduced vibration during engine re-starts and better vehicledrive feel.

CONDITION ACTION/FUNCTION Cold start cranking During such operation,advanced timing may be and run-up desirable, however, sufficienthydraulic pressure may not be available. Therefore, timing is advancedas soon as sufficient hydraulic pressure is present, assuming it ispossible to utilize retarded timing for cold starting operatingconditions. In other words, engine starts may be used with cam timing inthe full retard position, even under cold weather and high altitudeconditions. However, immediately after a cold start, engine speed can befairly high (~1200 RPM) and oil viscosity may also be high, so that VCToperation may be possible immediately after engine start. Cold startidle after During such operation, advanced timing may be initial run-updesirable. Therefore, engine idle speed is maintained high enough tomaintain sufficient oil conditions to enable control of the timingactuator (note that cold oil actually assists such operation byincreasing viscosity and thus pressure) Hot start cranking During suchoperation, place/leave actuator in fully and run-up retarded lockedposition to obtain improved hot restart with retarded timing. Hotstabilized idle During such operation, advanced timing may be desirable.Therefore, engine idle speed is maintained high enough via control ofthe transmission, and/or hybrid motor/generator. Alternatively, theengine may be deactivated if the desired speed is too low (whilecontinuing to operate the vehicle using the hybrid propulsion system).Shutdown During such operation, place/leave actuator in fully retardedlocked position to obtain improved shutdown operation. Low RPM wide-During such operation, advanced timing may be open throttle desirable.Therefore, engine idle speed is maintained high enough (e.g., at aminimum RPM) to enable valve timing advancement

Referring now to FIG. 1, an example hybrid propulsion system 11 for avehicle is shown including internal combustion engine 10, furtherdescribed herein with particular reference to FIGS. 2-3, and atransmission 15. In this example embodiment, the hybrid propulsionsystem 11 also includes a motor/generator 18 and an energy storagedevice 20. FIG. 1 shows generically that the engine, motor/generator,transmission, and/or energy storage device are interconnected. In oneexample, the system 11 may be coupled together in a starter/generatorconfiguration in which the motor/generator is coupled between the engine10 and transmission 15. Alternatively, the system 11 may be coupled in aparallel, series, or combined parallel-series configuration, such aswhere either the engine and/or the motor can drive the wheel 19, forexample.

The transmission 15 may be a manual transmission, automatictransmission, or combinations thereof. Further, various additionalcomponents may be included, such as a torque converter, and/or othergears such as a final drive unit, etc. Transmission 15 is shown coupledto drive wheel 19, which in turn is in contact with road surface 12.

The energy storage device 20 may include a battery, a capacitor, aflywheel, a hydraulic or pneumatic pressure vessel, among others andcombinations thereof. The motor/generator can be operated to absorbenergy from vehicle motion and/or the engine and convert the absorbedenergy to an energy form suitable for storage by the energy storagedevice. The motor/generator can also be operated to supply an output(power, work, torque, speed, etc.) to the drive wheels 19 and/or engine10 using stored energy.

In some embodiments, the motor may be configured to also serve as agenerator, thereby eliminating one or more separate generator devices.Alternatively, in some embodiments, a separate motor and generator canbe used where the motor is configured to provide a motor output from theenergy supplied by the battery, and the generator is configured toabsorb output (e.g. power, torque, work, speed, etc.) from the engineand/or transmission, and convert the absorbed output to energy storableby the energy storage device. The term motor will be used herein todescribe a device that can provide the role of both a generator and amotor.

Various types of energy/torque transmission may be used, such as amechanical coupling between the motor 18 and engine 10 or transmission15. Further, any connections between the motor and the energy storagedevice may indicate transmission of a variety of energy forms such aselectrical, mechanical, hydraulic, pneumatic, etc. For example, torquemay be transmitted from engine 10 to drive the vehicle drive wheels 19via transmission 15. As described above motor 18 may be configured tooperate in a generator mode and/or a motor mode. In a generator mode,system 18 absorbs some or all of the output from engine 10 and/ortransmission 15, which reduces the amount of drive output delivered tothe drive wheel 19, or the amount of braking torque to the drive wheel19. Such operation may be employed, for example, to achieve efficiencygains through regenerative braking, improved engine efficiency, etc.Further, the output received by system 11 may be used to charge energystorage device 20. In motor mode, the system 11 may supply mechanicaloutput to engine 10 and/or transmission 15, for example by usingelectrical energy stored in an electric battery.

As noted herein, hybrid propulsion embodiments may include full hybridsystems, in which the vehicle can run on just the engine, just themotor, or a combination of both. Assist or mild hybrid configurationsmay also be employed, in which the engine is the primary torque source,with the hybrid propulsion system acting to start the engine and toselectively deliver added torque, for example during tip-in or otherconditions. Further still, starter/generator and/or smart alternatorsystems may also be used. In any case, the hybrid propulsion system isable to utilize the motor to supply and/or absorb torque during vehicleoperation, such as conditions other than just engine starting ascompared to a conventional starter motor.

FIG. 2 shows one cylinder of a multi-cylinder engine, as well as theintake and exhaust path connected to that cylinder. Continuing with FIG.2, direct injection internal combustion engine 10, comprising aplurality of combustion chambers, is controlled by electronic enginecontroller 12. Combustion chamber 30 of engine 10 is shown includingcombustion chamber walls 32 with piston 36 positioned therein andconnected to crankshaft 40. A starter motor (not shown) is coupled tocrankshaft 40 via a flywheel, planetary gearset, accessory drive belt,or other linkage (not shown). Combustion chamber, or cylinder, 30 isshown communicating with intake manifold 44 and exhaust manifold 48 viarespective intake valves 52 a and 52 b (not shown), and exhaust valves54 a and 54 b (not shown). While in this example two intake and twoexhaust valves are used, alternative valve configurations may also beused, such as, for example, one intake and one exhaust valve, or twointake and one exhaust valves.

Fuel injector 66A is shown directly coupled to combustion chamber 30 fordelivering injected fuel directly therein in proportion to the pulsewidth of signal fpw received from controller 12 via electronic driver68. The fuel injector may be mounted in the side of the combustionchamber or in the top of the combustion chamber, for example. Fuel isdelivered to fuel injector 66A by a conventional high pressure fuelsystem (not shown) including a fuel tank, fuel pumps, and a fuel rail.

Intake manifold 44 is shown communicating with throttle body 58 viathrottle plate 62. In this particular example, throttle plate 62 iscoupled to electric motor 94 so that the position of throttle plate 62is controlled by controller 12 via electric motor 94. This configurationis commonly referred to as electronic throttle control (ETC), which isalso utilized during idle speed control. In an alternative embodiment(not shown), which is well known to those skilled in the art, a bypassair passageway is arranged in parallel with throttle plate 62 to controlinducted airflow during idle speed control via a control valvepositioned within the air passageway.

Exhaust gas sensor 76 is shown coupled to exhaust manifold 48 upstreamof catalytic converter 70. Sensor 76 may be any of many known sensorsfor providing an indication of exhaust gas air/fuel ratio such as alinear oxygen sensor or UEGO (universal or wide-range exhaust gasoxygen), a two-state oxygen sensor or EGO, a HEGO (heated EGO), a NOx,HC, or CO sensor.

Ignition system 88 provides an ignition spark to combustion chamber 30via spark plug 92 in response to spark advance signal SA from controller12, under select operating modes. Though spark ignition components areshown, engine 10 (or a portion of the cylinders thereof) may be operatedin a compression ignition mode, with or without spark assistance (and/oran additional injection to assist in commencing auto-ignition by raisingcylinder temperature). Further, in an alternative embodiment, thecombustion chamber has no spark plug.

Controller 12 may be configured to cause combustion chamber 30 tooperate in various combustion modes, as described herein. The fuelinjection timing may be varied to provide different combustion modes,along with other parameters, such as EGR, valve timing, valve operation,valve deactivation, etc.

The example exhaust emission control device 70 represents one or morecatalytic devices, such as three way catalyst, NOx traps, etc. that maybe used.

Controller 12 is shown in FIG. 2 as a conventional microcomputer,including microprocessor unit 102, input/output ports 104, an electronicstorage medium for executable programs and calibration values shown asread only memory chip 106 in this particular example, random accessmemory 108, keep alive memory 110, and a conventional data bus.Controller 12 is shown receiving various signals from sensors coupled toengine 10, in addition to those signals previously discussed, includingmeasurement of inducted mass air flow (MAF) from mass air flow sensor100 coupled to throttle body 58; engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling sleeve 114; a profile ignitionpickup signal (PIP) from Hall effect (or other type) sensor 118 coupledto crankshaft 40; and throttle position, TP, from throttle positionsensor 120; and absolute manifold pressure signal, MAP, from sensor 122.Engine speed signal, RPM, is generated by controller 12 from signal PIPin a conventional manner and manifold pressure signal MAP from amanifold pressure sensor provides an indication of vacuum, or pressure,in the intake manifold. Note that various combinations of the abovesensors may be used, such as a MAF sensor without a MAP sensor, or viceversa. During stoichiometric operation, this sensor can give anindication of engine torque. Further, this sensor, along with enginespeed, can provide an estimate of charge (including air) inducted intothe cylinder. In one example, sensor 118, which is also used as anengine speed sensor, produces a predetermined number of equally spacedpulses every revolution of the crankshaft.

In this particular example, temperature Tcat1 of device 70 may beinferred from engine operation. In an alternate embodiment, temperatureTcat1 is provided by temperature sensor 124.

Continuing with FIG. 2, engine 10 is shown with an intake camshaft 130and an exhaust camshaft 132, where camshaft 130 actuates both intakevalves 52 a,b and camshaft 132 actuates both exhaust valves 54 a,b. Thevalves can be actuated via lift profiles on the camshafts, where thelift profiles between the different valves may vary in height, duration,and/or timing.

For example, actuators 136 and 138 may vary the timing and/or lift ofcams 130 and 132, respectively. However, alternative camshaft (overheadand/or pushrod) arrangements could be used, if desired. In one example,actuators 136 and 138 are hydraulic vane type actuators in whichhydraulic engine oil (pressured by an engine oil pump) is used toadvance/retard the cam/valve timing. In some examples, a locking pin andspring mechanism are utilized to place the actuators in default lockedpositions if insufficient oil pressure/flow is present to control timingposition (e.g., at low speed conditions when the oil is warmed up,during engine stopped conditions, etc.). The locked position may beeither fully advanced, or fully retarded, for example.

In one example embodiment, where the full retard position is desired forwarmed-up shutdowns and re-starts, the fully locked position may beselected as full retard. However, the full retard position may not beappropriate for all cold start, idle, or low RPM wide-open throttleconditions, where oil pressure may also be low. Thus, there may beconflicting requirements for the design of the mechanical locking pinsfor VCT actuators. As such, various methods of engine operation may beutilized as described in more detail below herein. Alternatively,electrically actuated valves may be used. However, such mechanisms aregenerally more suited to double overhead cam engines and may increasecost.

Still further, mechanisms for variable cam timing that do not requirehigh oil pressure may be used, such as cam torque actuated VCTmechanisms. In yet another embodiment, a larger engine oil pump may beused to enable VCT operation at lower RPM, in which case the fullyadvanced or fully retarded position may be selected as the lockedposition. In still another embodiment, an electric engine oil pump maybe used to provide oil pressure even when the engine providesinsufficient oil pressure. I.e., an electric pump may be used in placeof, or in addition to, an engine driven oil pump.

Continuing with FIG. 2, it merely shows one cylinder of a multi-cylinderengine, and it is understood that each cylinder has its own set ofintake/exhaust valves, fuel injectors, spark plugs, etc. In analternative embodiment, a port fuel injection configuration may be usedwhere a fuel injector is coupled to intake manifold 44 in a port, ratherthan directly to cylinder 30.

Further, in the disclosed embodiments, an exhaust gas recirculation(EGR) system may be used to route a desired portion of exhaust gas fromexhaust manifold 48 to intake manifold 44 via an EGR valve (not shown).Alternatively, a portion of combustion gases may be retained in thecombustion chambers by controlling intake/exhaust valve timing.

The engine 10 may operate in various modes, including lean operation,rich operation, and “near stoichiometric” operation. “Nearstoichiometric” operation refers to oscillatory operation around thestoichiometric air fuel ratio.

Referring now to FIG. 3, an alternative valve configuration is shownwhere a single overhead camshaft 134 is used to actuate both intake andexhaust valves via respective rocker arms 146 and 148. The camshaft 134may be adjusted between an advanced and retarded position via variablevalve timing actuator 144, which may be a hydraulic actuator, forexample. This example may be referred to as dual equal variable camtiming, as any advance/retard affects both intake and exhaust valvetiming substantially equally. In other words, this example shows acommon camshaft which actuates at least one exhaust valve and one intakevalve and may have adjustable valve timing. In particular, the camtiming may be adjustable in a range of approximately 60 degrees crankangle. Thus, when the cam timing is retarded, each of the intake valveopening timing, intake valve closing timing, exhaust valve openingtiming, and exhaust valve closing timing are all retarded by anapproximately equal amount of degrees. Likewise, when the cam timing isadvanced, each of the intake valve opening timing, intake valve closingtiming, exhaust valve opening timing, and exhaust valve closing timingare all advanced by an approximately equal amount of degrees.

Note that FIGS. 2-3 shows just some examples of systems that canadvance/retard both an intake and an exhaust valve timing of a cylinder.For example, the range of adjustment may be greater or less than 60degrees, or may use various mechanisms to achieve unequal advance/retardbetween valves. Whatever the mechanism to provide retard of both intakeand exhaust valve events under selected conditions (e.g., 30 to 60degrees crank angle), such operation can provide improved fuel economyand reduced emissions at part load, and some benefits at wide-openthrottle.

For example, as shown with regard to FIG. 3A, the graph shows exampleintake and exhaust valve timing, along with an example range of valvetiming adjustment. In some prior examples, the locking pin engages inthe fully advanced position, where the fully advanced position is usedfor cold starts, warm restarts, idle, and low RPM wide-open throttle. Upto 60 degrees of retard may be used for improved part load fuel economyand feedgas emissions. In one embodiment of the present application,improved warm engine restart behavior can be attained by engaging thelocking pin in the fully retarded position. This can be combined withmodified control strategy for cold starts and/or idle and/or low RPMwide-open throttle, as described below.

Referring now to FIGS. 4-7, routines are described for controllingengine and hybrid propulsion system operation. Specifically, the controlroutines may be used for oil pressure actuated dual retard enginesystems in a hybrid propulsion system. The methods and actions describedin the flowcharts recognize the interactions between the engine andhybrid system with a variable cam timing actuator having a lockingposition in a retarded position (e.g., locking pins would hold the camsin the full retard for one or both of intake and exhaust valve timingwhen oil pressure is too low). As noted above, such retarded timing maybe advantageous for warmed-up shutdowns and re-starts.

However, full retard may not always be appropriate for warmed-up idle,low RPM wide-open throttle, or other low RPM conditions when the oil iswarm (typically oil pressure may be too low for VCT operation belowabout 700-900 RPM). In one embodiment, the hybrid system can be used toadjust system operation to avoid operation in some of the aboveconditions. For example, most hybrid systems seek to minimize or reduceoperation at warmed-up idle for improved fuel economy. However, even insuch systems, some idle operation may be used when air conditioning isrequired, when the energy storage system is low (e.g. low battery stateof charge), or for other reasons. Under these conditions, idle speed maybe increased as a function of measured or inferred oil pressure. Thus,rather than disabling VCT operation, idle speed may be increased as afunction of oil temperature (or inferred oil temperature), as describedin more detail below.

Further, operation below 700-900 RPM at higher loads is less common.Even so, in a hybrid with CVT-like (Continuously Variable Transmission)functionality, the control system can directly change engine speedwithout changing vehicle speed, and thus may use logic similar to thatabove to maintain engine speed above a minimum VCT operation enginespeed. In a hybrid system without CVT-like transmission functionality,minimum engine speed may be a constraint in the control logic for gearselection, torque converter lockup, and electronic throttle operation,etc., so that a minimum engine speed may be maintained when advancedtiming is needed.

Referring now specifically to FIG. 4, a routine is described foradjusting engine operation to maintain variable cam timing operation isdescribed. First, in 410, the routine determines whether idle conditionsare present. Such a determination may be based on, for example, enginespeed, engine power output, pedal position, vehicle speed, orcombinations thereof. If idle conditions are not present, the routinecontinues to 411. If idle conditions are present, the routine continuesto 412 to determine whether engine operation is required. For example,engine operation may be required due to low battery state of charge or aneed to operate an air-conditioning system.

If the answer to 412 is no, the routine continues to 414 where theengine is shut down and the vehicle is operated via the motor/generator18. Otherwise, when the answer to 412 is yes, the routine continues to416 to determine a desired engine speed. For example, the routine maydetermine a desired engine idle speed based on engine coolanttemperature, exhaust or catalyst temperature, ambient air temperature,accessory demand, amount of energy stored in device 20, or variouscombinations thereof. Next, the routine continues to step 418 todetermine whether the desired engine idle speed is less than a minimumengine speed for variable cam timing operation. The minimum speed forvariable cam timing operation may be based on hydraulic oil temperature,hydraulic oil pressure, engine temperature, or various combinationsthereof. Alternatively, in 418, the routine may determine whether thehydraulic oil pressure for controlling variable cam timing is less thana minimum hydraulic pressure for acceptable valve control.

If the answer to 418 is no, the routine continues to 422 to controlengine idle speed to the desired idle speed and adjust variable camtiming operation to the desired timing. For example, if the engine isoperating under cold idle conditions, the routine advances intake and/orexhaust valve timing.

Alternatively, when the answer to 418 is yes, the routine continues to424 to increase the desired engine idle speed to the minimum speed tomaintain variable cam timing operation. Then from 424, the routine alsocontinues 422.

In this way, operation may be adjusted to provide acceptable valvetiming operation, and engine shut down operation.

Referring now to FIG. 5, a routine is described for controlling enginespeed to a minimum acceptable speed for maintaining variable cam timingoperation outside of idle conditions. First, in 510, similar to 418, theroutine determines whether the engine speed is below a minimum speed formaintaining variable cam timing operation. If so, the routine continuesto 512 to increase engine speed at least to the minimum engine speed byadjusting transmission operation, motor operation, torque converterlockup, or combinations thereof. Referring now to FIG. 6, an engine shutdown operation is described. Specifically, in 610, the routinedetermines whether an engine shut down is in progress. If so, theroutine continues to 612 to move intake and exhaust valveopening/closing timings to a retarded, or fully retarded, position.

Referring now to FIG. 7, an example engine starting routine isdescribed. First in 710, the routine determines whether an engine startcondition is present. Various approaches may be determined to identifyan engine start, such as an engine start command from a vehicle systemcontroller, or an engine key on position, or various others. When theanswer to 710 is yes, the routine continues to 711 where the routinedetermines whether it is a hot restart or a cold start. For example, ahot restart may be one that is requested by the vehicle systemcontroller, while a cold start is one in response to a key on position.Alternatively, the two may be distinguished using exhaust or catalysttemperature, engine temperature, time since last start, etc. If a hotrestart is desired, the routine continues to 712 and starts the enginewith the intake and/or exhaust valve timing in a retarded, such as afull retard, position, in order to minimize oxygen flow to the catalyst.Next, in step 714, the routine determines whether sufficient oilpressure is present to control intake and/or exhaust valve timing awayfrom the locked position (e.g. full retard). If the answer to 714 isyes, the routine continues to 716 to adjust the cam timing to a desiredtiming based on engine operating conditions, such as engine coolanttemperature, number of combustion events from the engine start, timesince engine start, ambient temperature, barometric pressure, enginespeed, engine load, and various others.

Alternatively, if the answer to 711 is no, then a cold start is desiredand the routine proceeds to 718. In this case, the catalyst may besaturated with oxygen, and it may be preferable to start the engineafter the VCT is moved to a more advanced position. For example, a moreadvanced VCT position may improve combustion stability, improve fuelvaporization or air-fuel mixing, enable more spark retard and higherexhaust temperature, or reduce feedgas exhaust emissions. Therefore, instep 718 the routine increases engine speed without fueling or startingthe engine. In step 720 the routine checks whether target starting speedhas been reached, and whether oil pressure is high enough to enable VCToperation. If the answer to either question is no, the routine returnsto 718 and continues to increase engine speed. If the answer to 720 isyes, the routine continues to 722 where the VCT is moved to a moreadvanced position. When the VCT reaches the desired position for enginecold start, the routine continues to 724 where fuel and/or spark areenabled and the engine starts running. The routine then continues to 716where normal VCT operation is enabled.

In this way, it is possible to provide acceptable engine starting undera variety of operating conditions, and advantageously use anintake/exhaust variable valve adjustment system in which the valvetiming is locked in a retarded valve timing position for hot restarts,but moved to a more advanced position for cold starts.

The various embodiments and examples disclosed hereinabove haveaddressed a variety of issues related to engine starting/stopping,hybrid propulsion systems, and hydraulic VCT actuator systems.

For example, by using retarded intake and exhaust valve timing during atleast some engine shutdown operations and engine starting/crankingoperations, it is possible to reduce the flow of fresh air and/or oxygento the catalyst system, thus reducing a need for rich operation onrestarts (thus improving fuel economy) and reducing NOx and/orhydrocarbon emissions. Such operation is especially useful in a hybridsystem utilizing repeated engine stopping during vehicle operation, andthus may enable increased hybrid stop-start cycles, thus furtherimproving fuel economy and reducing emissions. Furthermore, the retardedintake/exhaust timing during shut-down and/or cranking/starting can alsoreduce the amount of trapped charge thus reducing torque pulses on thecrankshaft and improving NVH.

As another example, by appropriate selection of a variable valve timingactuator default position and system configuration, it is possible toaddress conflicting requirements for the default position for differentstarting conditions and different idle conditions, each of which rely onthe default position to control valve timing when insufficient oilpressure is present. Further, by utilizing the above-noted intake andexhaust valve timing controls in a hybrid propulsion system, it ispossible to take advantage of the ability to control engine speed atleast somewhat independently from vehicle speed, thereby reducing oravoiding selected operating conditions which may necessitate a defaultvalve timing position that degrades certain engine starting or idlingoperating conditions.

Referring now to FIGS. 8-12, addition control aspects related to enginestarting and/or stopping are presented.

In these examples, engine valve timing is adjusted to reduce airflow inresponse to a request to stop the engine, and fuel flow is stopped toselected cylinders having valve timing adjustments when the adjustedtiming reduces cylinder air charge of the cylinder below a predeterminedamount. By adjusting valve timing after a request to stop an engine andby deactivating fuel when the air amount inducted into a cylinder isbelow a level that likely supports a desired combustion stability level,engine emissions and undesirable operator perceptions may be reduced.For example, valve timing and cylinder fueling can be adjusted in acontrolled manner to reduce engine torque during an engine stopsequence, at least during some conditions. Further, stopping fuel flowwhen a cylinder inducted air amount reaches a predetermined level (e.g.,an air amount that can result in a desired level of likely combustionstability) can reduce engine emissions since engine misfires may bereduced, thereby decreasing the amount of exhausted hydrocarbons. Inaddition, audible engine noise and engine torque may be more uniformsince combustion may be more consistent. Reduced airflow may also reduceengine torque pulsations and NVH due to compression/expansion workduring shutdown and/or restart.

In another embodiment, a method to start a variable event valvetraininternal combustion engine may be used, comprising: increasing the speedof the engine during a start from a stopped position after a request tostart the engine; and increasing an intake valve timing amount of atleast a cylinder during the start. By increasing valve timing as enginespeed increases during an engine start, engine emissions and the amountof oxygen pumped to an exhaust system catalyst during engine startingmay be reduced. For example, by operating the valve timing (e.g., of adual retard engine) at a retarded position, the amount of air pumpedthrough the engine may be reduced. As engine speed increases, and asengine position is determined, valve timing may be advanced so thatcombustion may be initiated in selected cylinders. In this way, retardedtiming can reduce oxygen flow to a catalyst during a portion of astarting sequence and advanced timing can be used to increase cylindercharge so that torque can be generated during another portion of thestarting sequence. Reduced airflow may also reduce engine torquepulsations and NVH due to compression/expansion work during shutdownand/or restart.

In addition, during a start, fuel flow can be stopped until an inductedair amount reaches a level that reduces the chance of misfires. This mayfurther reduce engine starting emissions.

Referring now to FIG. 8, an example plot of signals that may be ofinterest during a simulated engine stop and/or start is shown. An enginestart may include a cranking period (Ref. FIG. 9), an assisted nearlyconstant rate of engine speed increase from a stop (Ref. FIG. 8( b)), ora cylinder initiated direct start. A starting interval may be defined ina number of ways including: a period between the point where enginerotation begins and when engine speed reaches a predetermined speed(e.g., idle speed); a period between the point where engine rotationbegins and when engine speed reaches a predetermined speed under powerof the engine; a period between the point where engine rotation beginsand when engine speed has passed through a predetermined speed apredetermined number of times; or a period between the point whereengine rotation begins and when engine speed has reached a predeterminedspeed for a predetermined period of time.

Graph (a) represents an example engine stop request signal. This signalmay be generated by an operator acting on a switch or automatically by acontroller that monitors vehicle operating conditions and determineswhen to stop and/or start the engine, a hybrid powertrain controller forexample. The high portion of the signal represents a command or requestto stop the engine while the low signal portion represents a request tostart the engine or to continue to operate the engine. The timing of theengine stop request relative to the other signals of FIG. 8 isillustrated by vertical lines T₁ and T₄.

Graph (b) illustrates an example engine speed trajectory during arequest to stop and start an engine.

In some hybrid vehicle configurations, engine speed may be controlledusing the secondary motor or independently from the secondary powerplant (e.g., an electric or hydraulic motor). U.S. Pat. Nos. 6,176,808and 6,364,807 describe a hybrid powertrain that may be capable ofcontrolling engine speed via a secondary motor and independent engineand motor speed control. The patents are hereby fully incorporated byreference. This engine speed trajectory represents one of severaltrajectories that may be possible by controlling engine speed in ahybrid powertrain. In one example, an electric motor and a transmissioncan be used to control engine speed during stopping and starting. Inaddition, the valve or cam phase angle may be controlled with respect tothe engine position and speed so that the inducted air amount may beregulated. In the figure, engine deceleration and acceleration arecontrolled during respective start and stop sequences. The engine speedand valve timing may be adjusted simultaneously to provide a desiredcylinder air amount.

Graph (c) shows three example cylinder air flow amounts over a number ofcombustion events during engine starting and stopping. During enginestopping a fixed cam mechanical valvetrain can induct air similar to theway that is described by line 402. Since the valve timing is fixed, thecylinder air flow may be largely a function of engine speed. Thecylinder air flow described by this line is the highest of the threeexamples. Cylinder air flow using a fixed cam mechanical valvetrainduring a start may be described by line segment 409. FIG. 8 shows theengine stop request at a low level T₄, indicating start and operate theengine, and engine speed increasing after the engine stop request hasbeen withdrawn. The cylinder and engine air flow increases as the enginespeed increases. If the cylinder air flow increases while combustion isinhibited, oxygen pumped through the engine may cool and/or occupycatalyst sites that may be used to reduce NOx. Consequently, theefficiency of the catalyst may be reduced. On the other hand, ifcombustion is initiated at low cylinder air flows misfires may result.Therefore, it may be desirable during a start to limit cylinder air flowand inhibit combustion until a desired level of combustion stability maybe attained.

Line 403 describes an example of cylinder air flow control using avariable event valve control mechanism that may be limited by certainphase control constraints. For example, a valve actuator may be limitedto a certain valve phase amount at a constant lift amount. The valveactuator phase amount control signal (e.g., of a dual retard system)described by the fourth graph (d) shows an example trajectory forreducing cylinder air flow during an engine stop. After a request tostop the engine, the valve phase may be adjusted to reduce cylinder airflow as shown in the graph (d), with lower values representing higherretard. The effect of engine speed and valve phase on cylinder air flowcan be seen in line 403 which shows two distinct segments that candescribe air flow during an engine stop. The first segment after arequest to stop the engine describes the effect of engine speedreduction and valve adjustments. The second distinct line segment occursafter the valve phase described by the graph (d) is complete (i.e., atsome altered phase amount). This line segment shows that the valveactuator phase limitations may not completely stop engine air flowthrough the engine while the engine is rotating, but that cylinder airflow can be reduced compared to a fixed timing mechanical valvetrain.

During engine starting, a phase limited valve actuator may be controlledsuch that the actuator can be indexed from a partial or minimum flowposition to another partial or full range flow position. By keeping theactuator at a minimum flow position the air flow through the engine maybe reduced during a start. For example, line 415 shows one possible airflow reduction strategy during starting. Cylinder air flow may bereduced while the engine speed is below a target or desired amount, andthen increased to a partial or full amount of the actuator range as theengine speed approaches a target speed, idle speed for example. Thisstrategy can lead to cylinder air flow that may be represented by thetwo segment line 415.

Cylinder air flow for a valve actuator that may be capable of reducingcylinder air flow to near zero during an engine stop may be described byline 401. This line shows an engine air flow amount that can be afunction of engine speed and valve phase. When the actuator reaches theminimum position illustrated in the graph (d), engine air flow isreduced to or near zero. Line 401 illustrates that it may possible toreduce the cylinder air amount to a level that is lower than the amountdescribed by line 402 (fixed cam valvetrain) and line 403 (limited rangevalve actuator).

Engine starting may be further improved by allowing reduced or no airflow through an engine during starting. As described above, air flowthrough an engine during starting can reduce catalyst efficiency. Line408 illustrates the result of one engine air flow amount controlstrategy that may be used to reduce the amount of oxygen that may bepumped to a catalyst during starting. Specifically, the air flow may belimited until a desired or target engine speed. Then, air flow may beincreased until a desired engine or cylinder air flow amount isachieved.

Graph (d) illustrates one example of a valve phase trajectory that maybe used to regulate engine and/or cylinder air flow. In this example,the valve phase command is reduced from an initial value (more advanced)at T₁ to a (retarded) value at T₃. Alternately, the phase reduction maybegin at a time before or after the engine request stop time. That is,the engine stop may be delayed until a predetermined valve phaseadjustment has been achieved, if desired. In addition, the valve phaseamount does not have to be linearly ramped to a reduced/retardedposition. Rather, the phase adjustment may be a step or steppedtransition, an exponential decay transition, or a transition that may bea combination of the previously mentioned methods.

As mentioned above, increasing valve phase during an engine start may bedelayed to reduce engine air flow. The engine start illustrated by FIG.8 delays the valve phase adjustment for the period between T₄ and T₅ andcompletes the adjustment by T₇ where the desired engine speed isreached. In this example the delay time before valve phase adjustment(T₅-T₄) can be determined from the amount of time it can take toaccelerate the engine from a stop to the desired start speed (T₄ to T₇),minus the time that it can take to move the valve phase actuator.Similar to the stop sequence, the valve phase amount does not have to belinearly ramped to a more advanced phase amount during a start. The liftmay be a step or stepped transition, an exponential rise transition, ora transition that may be a combination of the before mentioned methods.

As described above, depending on the valve actuator design, it may alsobe possible to adjust valve timing to control engine and cylinder airflow. Valve phase may be adjusted in the manner illustrated by graph(d). However, valve timing may be advanced or retarded to reduce theamount of engine air flow depending on the base valve timing and thephaser range of authority.

Graph (e) shows an example of fuel delivery control during enginestopping and starting. Fuel flow is stopped at T₂, a location that maybe coincident with an engine or cylinder air amount that designates alower boundary of air necessary for a desired level of combustionstability. That is, fuel flow may be stopped when combustion stabilityis likely to be less than a desired level, thereby reducing misfires. Inthis example, fuel can be stopped at a cylinder air amount identified atlocation 405 for a valvetrain that may be capable of reducing cylinderair flow to near zero, at location 406 for a valvetrain capable oflimited cylinder air flow reduction, and at location 407 for avalvetrain having fixed cam mechanically actuated valves.

Fuel control during a start is also shown in graph (e). Fuel may beenabled at T₆ where an increase in valve phase can allow a cylinder toinduct an air amount that may produce a desired level of combustionstability. Delaying fuel until a level of combustion stability may beattainable may reduce engine emissions and driver disturbances since thenumber of cylinder misfires may be reduced. In this example, fuel may bedelayed during a start for a cylinder air amount identified at location419 for a valvetrain that may be capable of reducing cylinder air flowto near zero, at location 411 for a valvetrain capable of limitedcylinder air flow reduction, and at location 413 for a valvetrain havinga fixed cam mechanically actuated valves.

An alternative method to start a variable event valvetrain can be toincrease engine speed from a stop to a predetermined speed (e.g., idlespeed) while the valve phase is set to a reduced amount and while fuelflow is stopped. At or near the predetermined desired engine speed, fuelflow may be activated and valve phase may be increased or valve phasemay be adjusted so that combustion may be initiated in one or morecylinders. In other words, at an engine stop, valve phase may initiallybe set to a fully retarded amount, and when the engine reaches apredetermined speed the valve phase may be advanced (partially or fully)and then fuel injection may proceed. Intake and/or exhaust valves may becontrolled in this manner, but engine starting may be more difficult ifflow through exhaust valves is reduced since more exhaust residuals maybe included in the cylinder mixture. In this way, valve timing canreduce or stop oxygen flow to an exhaust catalyst so that catalystefficiency may be increased. Reduced airflow may also reduce enginetorque pulsations and NVH due to compression/expansion work duringshutdown and/or restart.

Note that in the example of a hybrid powertrain, the system may have twoor more potential torque output devices and is may include thecombination of an internal combustion (IC) engine with a secondary powersystem. For example, a hybrid powertrain may comprise a combination ofan IC engine and an electric motor, an IC engine and a hydraulic powersystem, an IC engine and a pneumatic power system, an IC engine and oneor more energy storage flywheels, and various combinations of the beforementioned systems. In addition, during an engine stop it is notnecessary that the valve phase be adjusted from a maximum to a minimumamount. In other words, the valve phase can be reduced during the stopsequence from a first amount to a second amount. Also, the effect thatthe phase amount adjustment has on engine air flow may depend on enginespeed, valve geometry, and initial and/or final phase adjustmentamounts. Likewise, during an engine start it is not necessary toincrease the valve phase amount from a minimal amount to a maximumamount. The valve phase may be increased from a first amount to a secondamount. Furthermore, the valve phase of intake and exhaust valves mayalso be adjusted separately during an engine stop sequence.

Referring to FIG. 9, an example sequence that illustrates an alternativesimulation of an engine stop and start is shown. The signals and graphsare similar to those shown in FIG. 8. However, FIG. 9 illustrates adifferent engine starting method. In particular, engine starting withthe assistance of a starter motor is shown.

Graph (a) shows an example engine stop request signal. As mentionedabove, the request to stop may be generated in a number of waysincluding by a driver or by a hybrid powertrain controller. Graph (b)shows engine speed during a stop and a start. The engine stop sequenceis the same as in FIG. 8, but in this example no engine speed control isprovided by a large secondary motor (e.g., an electric or hydraulicmotor).

Engine starting speed is shown on the right hand side of the graph (b).The figure shows engine speed increasing and leveling off to a crankingspeed (i.e., the cranking period) by way of a starter motor. Crankingoccurs approximately during the period between T₄ and T₆. After fuel isintroduced at location T₆ the engine speed begins to increase from theresulting in- cylinder combustion. After run-up (i.e., the intervalbetween engine cranking speed and engine idle speed where the engine isaccelerating) the engine speed stabilizes at a predetermined level, idlespeed for example. However, it is not necessary that the engine speedremain at idle speed, the engine speed may change after the run-upperiod in response to operator demand.

Graph (c) shows cylinder air flow over a number of combustion eventsduring engine starting and stopping. Cylinder air flow lines 501, 502,and 503 show cylinder air flows for a valvetrain that can reduce flow toor near zero, a fixed cam mechanically driven valvetrain, and valvetrainactuator having a limited range of authority, respectively. Fuel flow isstopped at a cylinder air amount that is represented by the respectivecylinder air flow curves at locations 505, 507, and 506.

Similar to the sequence illustrated by FIG. 8, engine air amount can bereduced during a stop sequence so that combusted gases continue to heatand provide exhaust gases to a catalyst. The combusted gases flow to thecatalyst until a desired predetermined level of combustion stability maynot be attained. Further, air flow may be reduced until a desired valvephase is reached.

When starting by a starter cranking method, the cylinder air amount forrespective valvetrains may be illustrated by lines 508, 509, and 515.Cylinder air flow for a valvetrain having a fixed cam mechanicallyactuated valves corresponds to line 509, a valvetrain actuator havinglimited range of authority may be represented by line 515, and avalvetrain actuator capable of cylinder air flow to or near zero may berepresented by line 508. Fuel flow is started at a cylinder air amountthat is represented by the respective cylinder air flow curves atlocations 513, 516, and 511.

Graph (d) illustrates an example valve actuator phase amount duringengine stopping and starting. Cylinder air flow reduction by adjusting avalve actuator begins at T₁, coincident with the engine stop request,and ends at T₃.

On the right hand side of the graph (d), valve actuator adjustment isshown during a start. In this example, the valve adjustment is delayedfor a time after the request to stop the engine has been withdrawn. Thedelay period duration may be zero or it may be a function of the time torecognize engine position, engine position at start, time to pressurizethe fuel delivery system, engine temperature, or any other engine orvehicle operating condition, for example.

Graph (e) of FIG. 9 illustrates the timing of enabling fuel flow duringengine stopping and starting. During this example engine stoppingsequence, fuel is stopped at location T₂ which corresponds to a cylinderair charge at location 505 of the curve that represents one method ofcontrolling a valve actuator that may be capable of zero or near zerocylinder air flow. Locations 506 and 507 represent air charge amountsthat are equivalent to location 505 using different valve actuationmethods, but the time that it takes to achieve these levels of cylinderair charge may be increased since cylinder air amount is being reducedat a lower rate. Consequently, in other examples, fuel flow deactivationmay be delayed by the amount of time that it may take to reach thecylinder air amount that represents a desired level of combustionstability. This method can be used to decrease engine torque whileproviding a combusted mixture to the catalyst, and may reduce the amountof air that may be pumped to the catalyst during an engine stop. Reducedairflow may also reduce engine torque pulsations and NVH due tocompression/expansion work during shutdown and/or restart.

Fuel flow enablement during a start is shown by the right hand side ofgraph (e). At location T₆ fuel is activated, this location correspondsto the cylinder air amount 516 that can provide a desired level ofcombustion stability. Cylinder air amounts at locations 513 and 511 arethe same level of cylinder air amount at location 516, but the cylinderair charge levels are achieved before the time that the cylinder aircharge is achieved at location 516. In other words, during cranking andrun-up more air may flow through an engine having a fixed cammechanically actuated valvetrain or through a limited range adjustablevalvetrain than through a valve actuator that may be capable of zero ornear zero cylinder air flow. Reducing the air flow through the engineduring cranking and run-up may reduce engine emissions. For example,fuel may be delayed during a start so that the engine controller hastime to determine engine position and deliver a fuel amount to aselected cylinder. However, by delaying fuel flow during a start, somecylinders may pump air though the engine thus cooling and/or oxygenatingthe catalyst, thereby potentially reducing catalyst efficiency during asubsequent restart.

Referring to FIG. 10, a flow chart of an example engine stoppingsequence for a variable event valvetrain engine is shown. During anengine shutdown (i.e., an engine stop sequence) some engines are stoppedby immediately stopping fuel flow and spark to the engine cylinders.After fuel flow is stopped the engine can continue to rotate as theengine speed decreases. As a consequence, air that has not participatedin combustion may be pumped from the intake manifold to the exhaustsystem and through a catalyst. This may increase engine emissions whenthe engine is restarted since the air may cool the catalyst and/or theoxygen in air may occupy catalyst sites that otherwise could be used toreduce NOx.

In step 601, the routine determines if a request to stop the engine hasbeen made. If a request has not been made to stop the engine the routineexits. The routine of FIG. 10 can be repeatedly executed atpredetermined times or in response to an engine or controller operatingevent so that valve adjustments may be readily made. If a request hasbeen made the routine continues to step 602.

In step 602, engine speed can be reduced, and cylinder air flow may alsobe reduced by adjusting a valve actuator mechanism. In one embodiment,valve phase amount may be adjusted to reduce the cylinder charge mass,thereby, reducing the available cylinder torque. For example, the intakeand/or exhaust valve opening and/or closing positions relative to acrankshaft position may be adjusted to reduce the cylinder charge mass.The adjustments to valve phase may be made simultaneously orconsecutively. Fuel adjustment may be made proportionally to thecylinder air amount adjustment or it may be a function of engineoperating conditions, such as engine temperature and time since start,for example.

A number of different methods may be used to adjust the valve actuator(e.g., valve opening and/or closing phase) so that cylinder air chargeand/or engine torque may be lowered during an engine stop. In oneembodiment, the valve opening and closing positions may be retarded oradvanced by 100 crankshaft angle degrees per second, for example, sothat the inducted air charge may be lowered. In yet another embodiment,the valve lift may be adjusted in further response to engine operatingconditions, barometric pressure and/or desired torque for example.

In one example, intake valve timing may be adjusted while exhaust valvetiming can be fixed so that exhaust valve opening and closing positionsare known. In this example and other examples, the method described inU.S. patent application Ser. No. 10/805642 can be used to determinecylinder air amount after a request to stop an engine and theapplication is hereby fully incorporated by reference. Individualcylinder air amounts can be determined from cylinder pressure which canbe related to engine torques by the following equation:

${{IMEP}_{cyl}({bar})} = {( \frac{\Gamma_{brake} - ( {\Gamma_{{friction}\_{total}} + \Gamma_{{pumping}\_{total}} + \Gamma_{{accessories}\_{total}}} )}{{Num\_ cyl}_{Act}} )*\frac{4\pi}{V_{D}}*{\frac{( {1*10^{- 5}{bar}} )}{N\text{/}m^{2}} \cdot {SPKTR}}}$

Where IMEP_(cyl) is the cylinder indicated mean effective pressure,Γ_(brake) is engine brake torque, Γ_(friction) _(—) _(total) is thetotal engine friction torque, Γ_(pumping) _(—) _(total) is the totalengine pumping torque, Γ_(accessories) _(—) _(total) is the total engineaccessories torque, Num_cyl_(Act) is the number of active cylinders,V_(D) is the displacement volume of active cylinders, SPKTR is a torqueratio based on spark angle retarded from minimum best torque (MBT),i.e., the minimum amount of spark angle advance that produces the besttorque amount. By reducing the engine brake torque, engine speed may bereduced during a stop.

The term SPKTR can be based on the equation:

${SPKTR} = \frac{\Gamma_{\Delta\;{SPK}}}{\Gamma_{MBT}}$Where Γ_(ΔSPK) is the torque at a spark angle retarded from minimumspark for best torque (MBT), Γ_(MBT) is the torque at MBT. The value ofSPKTR can range from 0 to 1 depending on the spark retard from MBT.

Individual cylinder fuel mass can be determined, in one example, foreach cylinder by the following equation:m _(f) =C ₀ +C ₁ *N+C ₂ *AFR+C ₃ *AFR ² +C ₄ *IMEP+C ₅ *IMEP ² +C ₆*IMEP*NWhere m_(f) is mass of fuel, C₀-C₆ are stored, predetermined, regressedpolynomial coefficients, N is engine speed, AFR is the air-fuel ratio,and IMEP is indicated mean effective pressure. Additional or fewerpolynomial terms may be used in the regression based on the desiredcurve fit and strategy complexity. For example, polynomial terms forengine temperature, air charge temperature, and altitude might also beincluded.

A desired air charge can be determined from the desired fuel charge. Inone example, a predetermined air-fuel mixture (based on engine speed,temperature, and engine load), with or without exhaust gas sensorfeedback, can be used to determine a desired air-fuel ratio. Thedetermined fuel mass from above can be multiplied by the predetermineddesired air-fuel ratio to determine a desired cylinder air amount. Thedesired mass of air can be determined from the equation:m _(a) =m _(f) ·AFRWhere m_(a) is the desired mass of air entering a cylinder, m_(f) is thedesired mass of fuel entering a cylinder, and AFR is the desiredair-fuel ratio.

In one example, valve timing that can be used to induct the desiredamount of air into a cylinder may be determined by the method describedin U.S. Pat. No. 6,850,831 which is hereby fully incorporated byreference. Intake valve closing position can influence cylinder airamount, at least during some conditions, because inducted cylinder airamount can be related to the cylinder volume at IVC and the pressure inthe intake manifold. Therefore, the cylinder volume that can hold thedesired mass of air in the cylinder may be determined so that the IVClocation may be established. In other words, the cylinder volume duringthe intake and/or compression stroke that can hold the desired air mass,at a given intake manifold pressure, may be resolved into a uniquecrankshaft angle, the angle describing IVC. The cylinder volume at IVCfor a desired mass of air entering a cylinder may be described by thefollowing equation:

$V_{a,{IVC}} = \frac{m_{a}}{\rho_{a,{IVC}}}$Where ρ_(a,IVC) is the density of air at IVC, V_(a, Ivc) is the volumeof air in the cylinder at IVC. The density of air at IVC can bedetermined by adjusting the density of air to account for the change intemperature and pressure at IVC by the following equation:

$\rho_{a,{IVC}} = {\rho_{amb} \cdot \frac{T_{amb}}{T_{IVC}} \cdot \frac{P_{IVC}}{P_{amb}}}$Where ρ_(amb) is the density of air at ambient conditions, T_(amb) isambient temperature, T_(IVC) is the temperature of air at IVC, P_(IVC)is the pressure in the cylinder at IVC, and P_(amb) is ambient pressure.In one example, where IVC occurs before bottom-dead-center (BDC),pressure in the cylinder at IVC can be determined by differentiating theideal gas law forming the following equation:

${\overset{.}{P}}_{IVC} = \frac{{{\overset{.}{m}}_{cyl} \cdot R \cdot T} - {P_{IVC} \cdot \overset{.}{V}}}{V}$Where P_(IVC) is cylinder pressure, V is cylinder volume at a particularcrankshaft angle, R is the universal gas constant, and {dot over (m)} isflow rate into the cylinder estimated by:

${\overset{.}{m}}_{cyl} = {\frac{C_{D} \cdot {A_{value}(\Theta)} \cdot P_{run}}{\sqrt{R \cdot T}} \cdot ( \frac{P_{cyl}}{P_{run}} )^{\frac{1}{\gamma}} \cdot \sqrt{\frac{2 \cdot \gamma}{\gamma - 1} \cdot ( \frac{P_{IVC}}{P_{run}} )^{\frac{\gamma - 1}{\gamma}}}}$Where C_(D) is the valve coefficient of discharge, A_(valve)(θ) iseffective valve area as a function of crankshaft angle θ, P_(run) is themanifold runner pressure which can be assumed as manifold pressure atlower engine speeds, and γ is the ratio of specific heats. C_(D) iscalibratible and can be empirically determined.

The effective valve area, A_(valve)(θ), can vary depending on the valvelift amount. The valve lift profile can be combined with the valvedimensions to estimate the effective area, A_(valve)(θ), via thefollowing equation:A _(valve)(Θ)=L(Θ)·2·π·dWhere L(θ) is the valve lift amount that may be determined empiricallyby considering cylinder charge motion, combustion stability, minimumvalve opening and closing duration, and emissions.

The volume of intake mixture at IVC may be determined by the followingequation:

$V_{i,{IVC}} = \frac{V_{a,{IVC}} - {( {1 - F_{e}} ) \cdot V_{r,{IVC}}}}{f_{air}}$Where f_(air) is the proportion of air in the intake mixture, V_(a,IVC)is the cylinder volume occupied by air at IVC as describe above, andF_(e) is the fraction of burned gas in the exhaust manifold that can bedetermined by methods described in literature. For stoichiometric orrich conditions F_(e) can be set equal to one. F_(air) can be determinedfrom:

$f_{air} = \frac{1}{1 + \frac{1}{AFR} + F_{i}}$Where AFR is the air fuel ratio and F_(i) is the fraction of burned gasin the intake manifold. F_(i) can be estimated by methods described inliterature. The volume occupied by the total mixture at IVC can bedetermined by the equation:V _(IVC) =V _(i,IVC) −V _(cl) +V _(r,IVC)Where V_(cl) is the cylinder clearance volume, V_(r,IVC) is the residualvolume at IVC, and V_(lVC) is the total cylinder volume at IVC. Thevolume occupied by residual gas at IVC can be described by:

$V_{r,{IVC}} = {\frac{T_{IVC}}{T_{exh}} \cdot \frac{P_{exh}}{P_{IVC}} \cdot ( {V_{r,{EVC}} + V_{cl}} )}$Where T_(IVC) is the temperature at IVC that may be approximated by aregression of the form T_(IVC)=f(N,m_(f),θ_(OV)). Where N is enginespeed, m_(f) is fuel flow rate, and θ_(OV) valve overlap. T_(exh) istemperature in the exhaust manifold, P_(exh) is pressure in the exhaustmanifold, V_(cl) is cylinder clearance volume, P_(IVC) is pressure inthe cylinder at IVC, and V_(r,EVC) is the residual volume at EVC. In oneexample, where IVO is before EVC and where EVC and IVO are after TDC,V_(r,EVC) can be described by:

$V_{r,{EVC}} = {\int{\frac{A_{e}(\Theta)}{{A_{i}(\Theta)} + {A_{e}(\Theta)}}{\mathbb{d}{V(\Theta)}}}}$Where the integral is evaluated from IVO to EVC, and where A_(i) andA_(e) are the effective areas of the intake and exhaust valves forθε(θ_(IVO), θ_(EVC)) that may be determined in the same manner asdescribed above for A_(valve)(Θ). In this example, a predetermined valvelift can be used to describe an effective area of the intake valveopening. The intake valve area may be varied as a function of Θ so thatfor a certain cylinder temperature and pressure, a desired mass fractionof EGR may be trapped in a cylinder displacing a volume V_(r,EVC).

The cylinder volume minus the clearance volume at IVC can then be usedto determine intake valve closing position by solving the followingequation for θ:

$V_{\Theta} = {\frac{\pi\; B^{2}}{4}\lbrack {r + C - ( {{{C \cdot \cos}\;\Theta} + \sqrt{r^{2} - {{C^{2} \cdot \sin^{2}}\Theta}}} )} \rbrack}$In this way, valve lift, IVC, and IVO can be determined by accountingfor EGR and desired air amount.

In addition, engine fuel can also be adjusted in step 602 so that adesired exhaust air-fuel mixture may be achieved. During some conditionsthe exhaust gas air-fuel mixture may be lean while during otherconditions the mixture may be rich or stoichiometric. For example, if anengine is stopped after being warm and if there may be a higherprobability that the engine will restart, as with some hybrid vehicleapplications, the air-fuel mixture can be commanded to stoichiometry sothat the probability of disturbing an exhaust system catalyst may bereduced. The routine proceeds to step 603.

In step 603, a decision is made to continue reducing cylinder air amountor to proceed to a step that can stop fuel flow to the engine. If thevalve timing determined from step 602 inducts a cylinder air amount thatmay not be sufficient for a desired level of combustion stability theroutine proceeds to step 604. If the cylinder air amount may be above anamount that supports a desired level of combustion stability, theroutine returns to step 602.

In step 604, fuel flow to the engine or cylinder can be stopped. Becausecylinder air amount may be adjusted to a level that may be below adesired combustion stability limit, it can be desirable to stop fuelflow to the engine or to individual cylinders. Fuel flow may be stoppedwhen at least one cylinder air amount may be below a desired amount orfuel may be stopped in individual cylinders as the respective cylinderair amount may be reduced below a desired amount. If fuel flow isstopped on an individual cylinder basis, the valve phase may continue tobe adjusted in cylinders that may not be below a desired cylinder airamount.

Spark may also be deactivated in step 604, preferably after the latestair-fuel mixture is combusted. Spark may be deactivated immediatelyafter combusting the latest injected fuel or it may be deactivated aftera predetermined number of cylinder cycles. By delaying sparkdeactivation, it may be possible to combust fuel that may be drawn intothe cylinder from an intake manifold puddle, for example. The routinecontinues to step 605.

In step 605, valve phase can be evaluated to determine if furtheradjustments may be desired. If the valve phase is not at a desired lowflow position the routine returns to step 604 where further valveactuator adjustment may be commanded. If the valve phase is at a desiredlow flow position, the routine can proceed to step 606.

In step 606, valve phase can be held in a retard phase position.Typically, variable event valve actuators can be designed with a minimumphase position. In this position, the valve phase may be advanced orretarded relative to TDC, for example. Consequently, in this step, valveoperating commands can be structured based on the actuator design sothat a reduced flow, including zero flow, may pass through the cylinderas the engine decelerates to zero speed.

By commanding the valves to a phase that reduces cylinder flow, oxygenpumped through the engine to a catalyst may be reduced. As mentionedabove, reducing oxygen flow to a catalyst can improve engine emissionsduring a subsequent start since the catalyst state may maintain adesirable level of oxidants. By regulating the amount of oxygen that maybe stored in a catalyst, catalytic reaction sites may be available forboth oxidation and reduction reactions, thereby increasing thepossibility of converting HC, CO, and NOx during a subsequent restart.On the other hand, if the amount of oxygen stored on the catalyst isgreater than desired, the catalyst NOx reduction capacity may bediminished since some reduction sites may be occupied by oxygen. Reducedairflow may also reduce engine torque pulsations due tocompression/expansion work during shutdown and/or restart. The routineproceeds to step 607.

In step 607, engine speed is compared to a predetermined level. Ifengine speed is below a predetermined level, vlv_lim, the routine exits.When the routine exits, the valve actuators may be set to a desiredposition so that air flow and the cooling and the oxygen that it canbring to a catalyst may be reduced. If engine speed is above thepredetermined level, the routine returns to step 606.

Referring to FIG. 11, an example flow chart of an engine startingsequence for an engine having a variable event valvetrain is shown.

After an engine is stopped, oxygen flow to a catalyst can alter thecatalyst chemical or physical state so that engine emissions mayincrease during a subsequent restart. That is, it can be possible tostop an engine when catalyst chemistry may be favorable to convertinghydrocarbons, carbon monoxide, and oxides of nitrogen. However, allowingthe amount of oxygen stored in the catalyst to increase during an enginestop period or during the starting can reduce the catalyst NOxconversion efficiency since oxygen flow to a catalyst can reducecatalyst temperature and since stored oxygen may be preferentially usedto oxidize hydrocarbons and carbon monoxide. Consequently, NOx may passthrough the catalyst without being reduced because potential reductionsites may be occupied by oxygen that may have been pumped through theengine. The method of FIG. 11 may reduce engine emissions by reducingthe amount of oxygen pumped through an engine during a start. Reducedairflow may also reduce engine torque pulsations and NVH due tocompression/expansion work during shutdown and/or restart.

Continuing with FIG. 11, in step 701, the routine determines if arequest to start the engine has been made. If there has been no requestto start the engine the routine can exit. The routine of FIG. 11 canalso be repeatedly executed at predetermined times or in response to anengine or controller operating event so that valve adjustments may bereadily made. If there is a request to start the engine the routineproceeds to step 703.

During this step the valves may also be commanded to an initialposition, a predetermined valve phase (e.g., a retarded phase) whereflow through the cylinder may be reduced when the engine rotates, ifdesired. However, valves may be held in a low flow position (e.g.,closing all valves, closing intake valves, or closing exhaust valves)while the engine is stopped to further reduce oxygen flow to a catalyst.

In step 703, the routine increases engine speed and determines when tobegin adjusting valve phase. In one example, the electric motor of ahybrid vehicle uses at least a portion of the electric motor power torotate an internal combustion engine. The engine speed can be ramped upto a desired speed in a linear manner, if desired.

The valve adjustment timing schedule can be resolved by subtracting thetime for the valve actuator to move from an initial position to adesired position, vev_ΔT, from the time to accelerate the engine fromstop to a desired speed, ΔT. That is, the valve adjustment starting timecan be expressed by the following equation:T _(—) strt _(—) vlv=ΔT−vev _(—) ΔT

FIG. 8 can be used to illustrate this method of valve actuator control.The starting sequence begins at the time represented by vertical line T₄and the engine reaches a desired speed at time T₇. This is the time ΔT.The time to move the valve actuator to a desired position is the timebetween T₅ and T₇, vev_ΔT, and may be a function of engine oiltemperature and/or battery voltage, for example. The engine rotates fromT₄ to T₅ before the valve actuator begins to move to the desiredposition. In this way, the air flow through the engine during an enginestart may be reduced since the valves may be commanded to a low flowposition while the engine speed is increasing and cylinders may bepumping air through the engine. The routine continues to step 705.

It is also possible to adjust intake and exhaust valve phase separatelyduring a start so that air pumped through an engine may be reduced. Forexample, exhaust valve phase may be initially set to a retarded, orfully retarded position, and then increased as engine speed increases.By adjusting phase at lower engine speed, less air may be pumped intothe exhaust manifold for at least a portion of the starting interval. Asengine speed increases, and as engine position is determined, exhaustvalve phase may be advanced so that combusted gases may be expelled intothe exhaust system. This method may be more beneficial after a longerengine off period than after a shorter engine off period since fewerexhaust residuals may be trapped within the cylinder.

In step 705, engine speed continues to increase and the variable eventvalvetrain may be held at a constant actuator position. That is, thevalve may be held at a minimum or flow reducing position. This methodcan allow the engine to reach a desired speed with reduced cylinderflow. The routine proceeds to step 707.

In step 707, a decision can be made to begin adjusting the variableevent valve actuator. If the valve starting time has been exceeded theroutine proceeds to step 709. If not, the routine returns to step 705.

In step 709, the variable event valvetrain may be adjusted while enginespeed is being increased. Cylinder air flow may be increased byadjusting the valve phase. The adjustment may be at a constant orvariable rate depending on objectives. Furthermore, the adjustment ratemay be based on time (e.g., milli-meters per second) or engine speed.Alternatively, the valve phase may be adjusted to produce a desiredcylinder or engine torque or to induct a desired cylinder air charge.The routine proceeds to step 715.

In step 715, a decision can be made to continue valvetrain adjustment orto proceed to step 717 based on engine speed. If the engine speed isbelow a predetermined desired amount the routine returns to step 709. Ifthe engine speed is above a predetermined amount the routine continuesto step 717.

In step 717, a decision can be made to continue valvetrain adjustment orto proceed to step 713. If the variable event valvetrain is at a desiredposition the routine proceeds to step 713. If not, the routine returnsto step 709.

Note: steps 715 and 717 may be combined into a single step that allowsthe routine to proceed to step 717 if both the engine speed is at adesired level and if the variable event valvetrain is at a desiredphase. If not, the routine would return to step 709.

In step 713, cylinder fuel can be enabled and the variable eventvalvetrain can be held in position. By delaying fuel until a desiredamount of cylinder air flow may be present, misfires may be reduced.Further, delaying valve adjustment until the engine is at a desiredspeed can reduce the air pumped to a catalyst and may improve engineemissions during the restart.

Cylinder spark can also be reactivated in step 713 so that the injectedfuel can be combusted. The routine proceeds to exit.

In an alternate example, a valve adjustment timing schedule illustratedby FIG. 9 may be used. In this example, the engine can be rotated by astarter motor that may be capable of rotating the engine at lowerspeeds, 300 RPM or less for example.

The starting sequence begins at T₄ and the engine is at a desired speedat T₇. The time to adjust the valve actuator is shown between time T₅and time T₇. In this example, the valve actuator does not begin toadjust the valve phase until location T₅. The delay time between T₄ andT₅ may be related to the time that it can take to synchronize the enginecontroller to the engine position and/or the delay time may be afunction of engine oil temperature and/or battery voltage, enginefriction, engine speed, and/or another engine related variable. Asmentioned above, air flow through the engine during an engine start maybe reduced since the valve may be commanded to a reduced flow position.

In yet another embodiment, the valve adjustment may begin coincident ordelayed from initial engine rotation by a predetermined amount of time.When the valve actuator reaches a position that can support a desiredlevel of combustion stability and/or a cylinder inducts a desired airamount, the fuel may be enabled.

The method of FIG. 11 may also be extended to include throttle control.In particular, an electronic throttle may be held closed or at a fixedposition at a start until engine position is determined and/or until apredetermined valve phase amount may be achieved.

Note that the control routines included herein can be used with variousengine configurations, such as those described above. The specificroutine described herein may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts orfunction illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the features and advantages of theexample embodiments described herein, but is provided for ease ofillustration and description. One or more of the illustrated acts orfunctions may be repeatedly performed depending on the particularstrategy being used. Further, the described acts may graphicallyrepresent code to be programmed into the computer readable storagemedium in controller 12.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-4,V-6, I-3, I-4, I-6, V-8, V-10, V-12, opposed 4, and other engine types.Specifically, dual Retard VCT may be applicable to all engines includingSOHC or pushrod engines. Various valve timing ranges may used, such asup to IVC of approximately 120 deg after BDC. As another example,various other mechanisms may be used to control intake and/or exhaustvalve timing. Further still, various hybrid propulsion systems may beused, such as hybrid electric, hybrid hydraulic, or combinationsthereof. The subject matter of the present disclosure includes all noveland nonobvious combinations and subcombinations of the various systemsand configurations, and other features, functions, and/or propertiesdisclosed herein.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the disclosed features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application. Such claims, whether broader, narrower, equal, ordifferent in scope to the original claims, also are regarded as includedwithin the subject matter of the present disclosure.

1. A method for controlling engine operation of an engine with variableintake and exhaust valve timing, the engine coupled in a hybridpropulsion system, the method comprising: operating the engine withretarded intake opening and closing timing and retarded exhaust valveopening and closing timing during at least one engine shut-down; andoperating the engine with retarded intake opening and closing timing andretarded exhaust valve opening and closing timing during at least oneengine start, wherein said start follows said shut-down, and both saidshut-down and start occur during vehicle operation where the vehicle ispowered by a motor in the hybrid propulsion system.
 2. The method ofclaim 1 where said retarded intake and exhaust valve timing includesoperating with an intake valve opening more than 20 degrees after topdead center.
 3. The method of claim 2 where said retarded exhaust valvetiming includes operating with an exhaust valve closing more than 20degrees after top dead center.
 4. The method of claim 1 wherein saidhybrid propulsion system is a hybrid electric propulsion system, andsaid shut-down and start are independent of a driver start/stop command.